1. Technical Field
The present invention relates to a pulse amplifier and a pulse light source using the same, and more specifically to the pulse light source suitable to use not only for a telecommunication sector but also for a non-telecom sector wherein a pulse of large peak power is required especially, such as a micro processing, a light source for generating a terahertz wave, a measurement, a biotechnological application, a multiphoton microscope, or the like.
2. Related Arts
Regarding such the pulse light source, an ultrashort pulse light source of a fiber type is attractive for an industrial application, because it is superior to a solid state laser from points of view of a small sized packaging, an operability, and robustness to the environment of such as dust, temperature, humidity, vibration, or the like. Moreover, regarding such the pulse light source, some developments are progressed for a shorter pulse and a higher pulse energy.
It is important to suppress a nonlinear chirp for progressing such the shorter pulse and the higher pulse energy regarding the pulse light source, because the nonlinear chirp causes a worse pulse quality. Moreover, an effect of a self phase modulation (SPM), which is one of nonlinear phenomena in an optical fiber, becomes larger as the peak power of the pulse becomes larger due to the shorter pulse or the higher pulse energy. Furthermore, due to such the SPM, a phase shift proportional to a time differential of a light intensity is to be additionally generated in the pulse, and generally a nonlinear chirp is to be additionally generated therein.
However, in the case of being the light intensity parabolic functional the additionally generated chirp becomes to be linear functional even in the case of the SPM occurring, because the time differential of the light intensity is a linear function. Therefore, a parabolic functional pulse is an ideal waveform for progressing the shorter pulse and the higher pulse energy. Such the parabolic functional pulse is to be changed similarly in pulse waveform thereof, at the period of propagating through a normal dispersion gain medium. And, such the pulse is called a similariton.
However, generally it is hard to generate such the ideal parabolic functional pulse, and the following conventional technologies are known therefor.
1. A technology for generating a similariton pulse (a parabolic functional pulse) using an ytterbium (Yb) doped fiber amplifier (Yb-DFA), with using an input pulse with a pulse width of several hundreds femtoseconds (fs), such as 200 fs for example (refer to a nonpatent document 1 for example).
2. A technology for generating a parabolic functional pulse using a Raman amplifier with a length of 6 km, with using an input pulse with the pulse width of ten picoseconds (ps) output from a gain-switched light source (refer to a nonpatent document 2 for example).
3. A technology for generating a parabolic functional pulse using an erbium (Er) doped fiber (EDF) with the length of 1.2 km, with using an input pulse with the pulse width of 2.4 ps (refer to a nonpatent document 3 for example).
4. A technology for generating a parabolic functional pulse using an EDF and a highly non-linear fiber (HNLF), with using an input pulse with the pulse width of 1.4 ps generated by a mode-locked laser (refer to a nonpatent document 4 for example).
5. A technology for generating a parabolic functional pulse using a comb-like dispersion profiled fiber (CDPF) comprised of six types of fibers having different dispersion characteristics, with using an input pulse with the pulse width of 190 fs (refer to a nonpatent document 5 for example).
6. A technology for generating a parabolic functional pulse by using only a normal dispersion fiber (refer to a nonpatent document 6 for example).    Nonpatent document 1: M. F. Fermann, CLEO2000, CME2.    Nonpatent document 2: C. Billet et al., CLEO-EP2003, CL6-1-FRI.    Nonpatent document 3: Y. Ozeki et al., Electron. Lett., vol. 40, p. 1103 (2004).    Nonpatent document 4: B. Kibler, Photon. Technol. Lett., vol. 18, p. 1831 (2006).    Nonpatent document 5: B. Kibler et al., Electron. Lett., vol. 42, p. 965 (2006).    Nonpatent document 6: C. Finot et al., OFC2007, OTuJ3.    Nonpatent document 7: Y. Ozeki et al., CLEO2004, CTuBB5.
However, such the abovementioned conventional technologies include the following disadvantages. According to the technology regarding the nonpatent document 1, a seed pulse light source tends to be limited to a narrow pulse width. Moreover, characteristics of an amplification medium tend to be limited therein. According to the technology regarding the nonpatent document 2, the Raman amplifier has an insufficient conversion efficiency, and then the quite long Raman amplifier with the length of 6 km is required. Such the Raman amplifier of 6 km is not suitable for the small sized packaging as it is difficult to store. According to the technology regarding the nonpatent document 3, the EDF with the extraordinary long length of 1.2 km comparing to an ordinary EDF with the length of several meters to several tens meters. It is hard to store such the EDF of 1.2 km. Moreover, a total cost becomes higher because such the EDF of 1.2 km is extremely expensive. According to the technology regarding the nonpatent document 4, a configuration is used in which the EDF and the HNLF are comprised, and the HNLF is used just for a nonlinear medium with no expectation of an effect as a normal dispersion medium. According to the technology regarding the nonpatent document 5, using six types of fibers causes the higher cost.
Moreover, according to the technology regarding the nonpatent document 6, a pulse is amplified to be with the optimum pulse energy for generating a parabolic functional pulse using an erbium doped fiber amplifier (EDFA), and then the parabolic functional pulse is to be generated by using a nonlinear effect and a dispersion effect in the normal dispersion fiber. However, such the method is not suitable to spread a spectrum because a peak power is to be decreased due to a dispersion during a propagation thereof. Furthermore, the EDFA is used just as the amplification medium without suggesting a use of an SPM effect in the EDFA.
Furthermore, it is necessary to balance optimally on a gain, a normal dispersion and an SPM, for amplifying a pulse with maintaining the pulse having a linear chirp. However, it is not so easy to provide the optimum medium therefor.
That is to say, the following input pulse is regarded as the optimum, for generating ideal parabolic functional pulse regarding the normal dispersion EDFA (refer to the above mentioned nonpatent document 7).T0=1.64√(β2/g),Ein=0.38√(β2g)/γ.
Here, the T0 designates a half width of the input pulse, the Ein shows a pulse energy of the input pulse, the β2 designates a second order dispersion, the γ designates a nonlinear coefficient, and the g designates a gain coefficient. For example, in the case of assuming 1.1 dB/m=253/km for the gain coefficient of an EDF and a full width at half maximum of 2 ps (the half width of 1.20 ps) for the input pulse, it becomes necessary to provide the EDF of extraordinary large normal dispersion as it is almost impossible to obtain, because the required dispersion for the EDF is estimated to be 136 ps2/km=−106 ps/nm/km thereby. That is to say, it is hard in particular to generate a parabolic functional pulse from a light pulse with the pulse width of not less than two picoseconds approximately with using such the normal dispersion EDF.